The invention relates to a pressure limiting valve for hydraulic support in underground mining and tunnel construction, with a tubular valve housing, the valve case that is movable in the guide against the force of the valve spring arranged and adjustable inside the valve housing, and the crossbores constructed in the valve housing or guide in front of the spring chamber, with the pressure medium being able to spray off through these crossbores when the valve responds, while bypassing the spring chamber.
Such pressure limiting valves are known from DE-OS 35 08 986.5-12. These known pressure limiting valves are used to protect underground support from overloads. Such overloads occur especially if the support is compressed due to convergence, causing the pressure in the individual props to rise. To avoid an overload in this case, part of the pressure fluid in the props is exactly discharged by the pressure limiting valves when a critical pressure of, e.g., above 400 bar is present. For this purpose, the pressure limiting valves are equipped with a valve spring that is arranged in the valve housing between the spring plate set onto the valve piston on the one side and the adjustment screw on the other side. If an overload occurs, the valve spring is then compressed by the valve piston and spring plate, so that the valve piston moves in the valve bore and is able to drive over an O-ring in the process, so that the pressure medium is then able to flow outward from the prop through the pressure limiting valve. To enable this flowing-off, the known valve has a bore in the adjustment screw or in the top part of the valve housing, resulting in an advantageous upward thrust effect that significantly increases the response accuracy of the valve. The spring itself is adjusted by turning the adjustment screw so that the valve accordingly opens at the predefined pressure. Regulations concerning operating and environmental protection reasons in underground mining and tunnel construction for such pressure limiting valves require that the discharged pressure medium must be collected and returned via a return line into the system. The pressure present in this return line and the pressure occurring during the flowing through the valve housing influence the adjustment force of the spring, so that an exact setting of the opening point is difficult.
DE-GM 84 08 072 introduces a pressure limiting valve where the spring chamber is kept dry by an extended guide in which crossbores are constructed, said crossbores extending in front of the spring chamber at a right angle to the valve bore and removing the pressure medium when the valve responds, while bypassing the spring chamber. The valve piston is divided into two pans, whereby the bottom valve piston has a smaller diameter than the top part of the valve piston that has no bores. Because of this, the two pans of the valve piston can be moved inside the valve bore when an overload occurs, whereby the pressure medium can be discharged through the crossbores in the area of the bottom piston part after the O-ring has been passed. When connecting a return line to the crossbores, but even without them, a back pressure would be generated during the spraying off of the pressure medium in this valve also, whereby this back pressure changes the set valve force to such an extent that the valve is no longer able to respond accurately.